1. The Field of the Invention
This invention relates to control methods and apparatus for use in steam boilers to prevent activation of the water heating elements in unsafe conditions.
2. Description of the Prior Art
Most steam boilers include a reservoir, in which the water is heated by the burner and a safety control system having a float mechanism for deactivating, or preventing activation of the burner, in the event the water level drops below a pre-determined danger, or cut-off point. The float is normally connected to the control system, by a pivotable linkage surrounded by an accordian-type protective sleeve, or bellows.
During extended periods of operation "mud" or "sludge" and other residue from the reservoir water accumulate in the float chamber and can prevent movement of the float, in response to changes in the water level. The mud can also accumulate on the bellows and prevent movement of the linkage. Furthermore, although the linkage itself is insulated from the mudd, it may become "frozen" by corrosion at the pivot points from the seepage of sediment through the bellows which would prevent the float from moving in response to changes in the water level. In any of these situations, the float may become stuck in an upper position above the water level, and, thus provide, no indication when the water level reaches the predetermined cut-off level, and therefore, not deactivate or prevent activation of the burner. This invariably results in all the water being boiled off and destructive failure, if not explosion of the furnace from the unsafe overheated condition.
Normal maintenance of these water-boiling furnaces requires that the blow-down valve lying underneath the low water shut-off valve be opened manually, on a regular basis in order to "flush" the mud, sludge or other sediment which accumulates in the float chamber. However, it has been well established that the control systems are infrequently "flushed" and thus, dry-boiler failures are not uncommon.
In the past, attempts to remedy this problem have been undertaken in one of two different ways.
First of all, attempts have been made to redesign the low water cut-off valve. However, such attempts have met with very little, if any commercial success, because of the expense of redesigning such valves, their cumbersom nature, and or their technical failure. One example of such an attempt was disclosed in U.S. Pat. No. 3,776,200. Therein, the traditional low-water shut-off valve is redesigned to accommodate a small motor with a cam which automatically pushes down the float to its lowest position just prior to each firing cycle of the boiler. If the float is either defective or so sticky that it does not return to its original position an alarm sounds and the boiler is prevented from firing. However, if the float releases to its original position the firing cycle goes forward, however, such a system has several drawbacks. That is the system does not alleviate the need for manually flushing the float chamber. It merely provides an indication of when maintenance is needed. Also, if, for some reason, there is not sufficient accumulation of sediment to restrict the float during the initial checking stage, but sufficient sediment accumulates during the firing cycle to cause the float to stick, the water level might still fall below the danger point and result in dry boiler failure. Additionally, the insulation of this system requires dismantling of currently employed low water shut-off valves and replacing them with the newly designed valve and the expense of such valves is so exorbitant that it is not commercially feasible in many places where boilers are already installed nor is it a commercially attractive alternative to the more simple low water shut-off valve, given the fact that maintenance is still required to prevent low water failure.
The second alternative approach to preventing low water or dry boiler failure is that of incorporating an internal water level probe into the interior jacket of the boiler reservoir. This probe is purportedly designed to detect water and upon water falling below the level of the probe, the circuit is broken and the burner of the boiler is deactivated. However, a primary drawback to this system, is that sediment, dirt, or mudd from the water often is stuck in the probe but remains moist enough to provide a false signal that the water level has not receded below the level of the probe. Accordingly, the burners of the boiler may continue to fire even though the water level has dropped below the probe and dry boiler failure occurs. Such systems are disclosed in, for example, U.S. Pat. No. 3834357. An additional drawback to such a system, is that although it provides a secondary backup for proportedly shutting down the boiler at low water points, these systems do not serve as a substitute for maintaining the low water control valve by way of manually flushing it.
Accordingly, there would be a substantial advancement and a satisfaction of a long-felt need in the industry to provide an inexpensive system for preventing dry-boiler failure which would eliminate the need for manually blowing down, which could be inexpensively and readily adapted to, and/or installed upon currently existing low water shut-off valves, and which has a back-up shut down of the boiler triggered upon overheating of the boiler reservoir rather than based upon a faulty water level monitoring probe.